The invention relates to a semiconductor device with a semiconductor body comprising a surface region of a first conductivity type which adjoins a surface and in which a field effect transistor with insulated gate is provided, with source and drain regions of the second, opposed conductivity type in the surface region and a drain extension region of the second conductivity type which adjoins the drain region and the surface, which has a lower doping concentration than the drain region, and which extends longitudinally in the direction of the source region, with a channel region of the first conductivity type situated between the drain extension region and the source region, and with a gate electrode situated above the channel region and separated from the channel region by an insulating layer.
Such a semiconductor device is particularly useful as a switching element at high voltages, for example, for applications in motorcars, TV sets and audio power amplifiers.
European Patent no. 69429 discloses a device of the kind mentioned in the opening paragraph. In this device, a drain extension region of the same conductivity type as but with a lower doping concentration than the strongly doped drain region is provided between the gate electrode and the comparatively strongly doped drain region. The surface region is electrically interconnected with the source region. Starting from a certain drain voltage, i.e. the voltage between source and drain regions, when the pn junction between the surface region and the drain extension region is blocked, the drain extension region is pinched off owing to an expansion of a depletion zone from the blocked pn junction, whereby the surface field strength at the side of the drain region is reduced and the voltage between the source and drain regions at which breakdown takes place, the so-called drain breakdown voltage, is increased.
Although high drain breakdown voltages can be achieved in this manner, a high breakdown voltage is accompanied in practice by a comparatively high source-drain resistance, i.e. the on-resistance (R.sub.on) of the transistor is high. This is because only a comparatively weak doping of the drain extension region will lead to a sufficient depletion of the drain extension region. This weak doping, however, leads to a high resistance of the drain extension region. In practice, moreover, an additional doping step is often necessary for providing the desired weak doping in the drain extension region. Such an additional doping step makes the process flow more intricate and expensive, and is therefore undesirable.